The present invention relates to the use of degradable fibers, and more particularly, to self-degrading fibers and their associated methods of manufacture and use in subterranean applications.
Degradable materials are increasingly becoming of interest in various subterranean applications based, at least in part, on their ability to degrade or produce desirable degradation products (e.g., acids). One particular degradable material that has received recent attention is poly(lactic acid) (“PLA”) because it is a material that will degrade down hole after it has performed a desired function or because its degradation products will perform a desired function (e.g., degrade an acid soluble component).
This invention relates to degradable fibers that may be used in any subterranean application wherein it is desirable to include degradable fibers, for instance, to create voids or produce associated degradation products. Examples of suitable applications include cementing, fracturing, and gravel packing, as well as other applications wherein it may be desirable to produce voids or particular degradation products through the degradation of self-degrading fibers.
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like. In additional applications, hydraulic cement compositions may be used in fracturing and gravel packing applications to form packs that are similar to gravel packs or proppant packs.
Hydraulic fracturing techniques are commonly used to stimulate subterranean formations to enhance the production of desirable fluids therefrom. In a conventional hydraulic fracturing process, a fracturing fluid is pumped down a well bore and into a fluid-bearing formation. The fracturing fluid is pumped into the formation under a pressure sufficient to create or enlarge fissures in the formation. Fracturing fluids used in conventional hydraulic fracturing techniques include: fresh water, brine, liquid hydrocarbons, gelled water, or gelled brine. The fracturing fluid may contain a viscosifying or gelling agent to increase its viscosity. The fracturing fluid typically also will contain a proppant that will be deposited in the fractures. Commonly used proppant particulates include particulate materials like sand, walnut shells, glass beads, metal pellets, and ceramic beads. The deposited proppant particulates often form proppant packs in the fractures to help to maintain the integrity of those fractures in the formation.
There have been attempts to use cement compositions as propping agents. Cement compositions are desirable in this application because of their high strength and low cost. In conventional methods, such cement compositions when used as propping agents often contain particulate carbonate salts. In theory, when the carbonate salts are removed from the cement composition at some point before the cement composition develops substantial compressive strength, the resultant cement matrix has some degree of permeability, which allows formation fluids to flow to the well bore. Carbonate salts, however, require an acid to dissolve out of the cement composition. Acid treatment may be unreliable because acid tends to find the path of least resistance within the cement composition, which results in uneven distribution of acid and resultant removal of carbonate salt particulates. Thus, the resultant permeability usually is not sufficient for hydrocarbon production. Moreover, the use of acid undermines the integrity of the cement by destabilizing the structure of the cement matrix, thus weakening the cement strength or consolidation.
Additionally, oil, gas, and water producing wells often are completed in unconsolidated subterranean formations containing loose or incompetent sands that can flow into the well bores with produced fluids. The presence of this sand in the produced fluids is undesirable as it, inter alia, may erode equipment, which often substantially increases the costs associated with operating such wells and generally reduces the fluid production capability of the formation. Incompetent subterranean formations include those which contain loose sand that is readily entrained by produced fluids, and those wherein the bonded sand particles comprising the formations lack sufficient bond strength to withstand the forces produced by the intermittent production of fluids from the formations.
Heretofore, unconsolidated formations have been treated by creating fractures in the formations and depositing proppant material, e.g., sand of a selected size, in the fractures to substantially preserve the fractures. In addition, the proppant has heretofore been consolidated within the fractures into hard permeable masses to prevent the proppant from flowing back and to reduce the migration of sand through the fractures with produced fluids. Further, costly “gravel packs,” which may include sand screens, slotted liners, perforated shrouds, and the like, have been utilized in wells to prevent the production of formation sand. In conventional gravel packing operations, graded sand is placed in the annulus between a screen and the walls of the well bore in the producing interval. The resulting structure provides a barrier to migrating sand while allowing desired fluids to flow into the well bore so that they may be produced.
While gravel packs may prevent the production of sand with formation fluids, they often fail and require replacement. This may be due to, for example, the deterioration of the screen as a result of corrosion or the like. The initial installation of a gravel pack adds considerable expense to the cost of completing a well, and the removal and replacement of a failed gravel pack is even more costly.
In horizontal well bores formed in unconsolidated formations, the well bores are often completed open hole, e.g., a casing is not inserted into the well bore. In open hole well bores, oftentimes a slotted liner, sand control screen, gravel pack, or the like is installed into the uncased well bore. This method of completion may be problematic as discussed above in that as the incompetent formation tends to break down as a result of production, the slotted liner, sand control screen, or gravel pack is often bypassed, which may result in formation sand being produced along with formation fluids.
There have been attempts to use a sort of permeable cement in subsurface applications such as gravel packs wherein the permeable cement composition contains a particulate, such as a carbonate salt or oil-soluble resin particulate, that is dissolvable with the addition of a second fluid, e.g., an acid or a hydrocarbon. The thought behind this approach is generally that when the dissolvable particulate dissolves out of the cement mass, voids are left in the cement mass so that the cement mass has some degree of permeability to formation fluids. Such permeable cement compositions and methods, however, have not been successful because the permeability of the cement mass once the particulate is dissolved out has not been satisfactory. This lack of permeability is caused by, inter alia, the dissolvable particulate's dependence on contact with a second solvent. Oftentimes, the solvent is not able to interact with a sufficient amount of the dissolvable particulate to adequately dissolve a sufficient amount of the particulate. As a result, not enough of the particulate is dissolved out of the cement mass to make the cement mass's permeability suitable for subsurface applications such as gravel packing.